Field
The present invention relates to a solid state fermentation process for producing methane, and to a bioreactor and solid support for use in said process.
Description of the Related Art
Methane (CH4) is a simple alkane hydrocarbon and the main component of natural gas. It is an attractive fuel, which fits well to the existing infrastructure. For instance, it may be used directly to heat homes and commercial buildings by feeding into the existing gas network which in many countries has one or two years of gas storage capacity. Methane may also be used in the generation of electric power or as a transportation fuel in gas vehicles.
Methane may be produced by reacting carbon dioxide and hydrogen in a Sabatier reaction: CO2+4H2→CH4+2H2O. The reaction may be catalyzed by two alternative ways: inorganically using metal catalysts at temperatures of several hundred degrees Celsius, or microbiologically at some tens of degrees Celsius.
Owing to the very high operating temperature required and the explosive nature of hydrogen, methane production in inorganic catalysers is a challenging task. Furthermore, required temperature control consumes energy thereby reducing the net efficiency of the system. These drawbacks can be avoided by using microbiologically catalyzed methane fermenting bioreactors.
General environmental factors affecting microbial activity in any bioreactor include water content, temperature, pH, partial pressure of dissolved oxygen and other gases, nutritional conditions, and degree of homogeneity. Traditionally, fermentation processes are carried out either in liquid or on moist solid particles. Mechanical agitation or stirring is the most common way of enhancing the transfer of gases and other substances in the bioreactor. Liquid fermentation coupled with agitation provides bioreactors that are easy to control. However, such bioreactors are expensive and agitation consumes high amounts of energy. If the bioreaction uses gaseous substrates and/or produces gaseous end products, securing efficient gas transfer at low cost becomes extremely difficult. Furthermore, formation of waste water in liquid fermentors may become a particular problem, especially, if the bioreaction produces water.
Solid-state fermentation processes provide several advantages over liquid fermentation processes. For instance, water which is a prerequisite for microbial growth exists mainly as adsorbed into or bound capillarily to the moist solid particles in the solid-state bioreactors. Thus, the water phase in the spaces between the particles is discontinuous and most of the inter-particle space is filled by the gas phase. This makes it relatively easy to feed gaseous starting materials into the bioreactor by applying pressure. In addition, any gaseous end products may exit the system by pressure differences. No agitation is needed in solid-state bioreactors and, thus, instrumentation may be far simpler than in liquid bioreactors. Furthermore, remarkably dense microbial growth on the moist solid particles may be achieved, resulting in high fermentation efficiency. The solid-state approach is particularly suitable for large-scale fermentation processes and bioreactors in cases where the unit prices of the end products are low and, thus, the aim is to build low-cost bioreactors with low maintenance costs.
There are some disadvantages associated with solid-state fermentation, too. For instance, owing to varying physical and chemical environmental conditions, the microbial growth and its efficacy may be unevenly distributed over the solid particles. Since the solid-state bioreactors cannot be homogenized by stirring, the availability of nutrients to the microorganisms may be uneven and it may be difficult to provide pH control. Furthermore, aeration or transfer of gaseous substances between different parts of the bioreactor may be limited. This may, for instance, be due to a blockade of the inter-particle space by condensing water, or water produced in the bioreaction. On the other hand, in cases where the bioreaction does not produce water, the solid particles may desiccate owing to gravity or gas flows, thus lowering the fermentation capacity of the microorganisms.
There have been attempts to produce methane in solid-state bioreactors. For instance, Jee et al. reported in Biotechnology Letters (1988, Vol 10: 243-248) that efficient CH4 production from H2 and CO2 could be achieved by fixing methanogen cells on a solid support such as porous ceramic. However, in long term operation the accumulation of methanogen cells on the support hindered the homogeneous flow of the gaseous substrates through the pores of the support and this caused a gradual decrease of methanation from H2 and CO2.
The present invention aims at avoiding disadvantages of conventional solid-state bioreactors, especially when the bioreaction involves gaseous starting materials and/or reaction products, and low building and maintenance costs are desired.